1. Field of the Invention
The present invention relates generally to a cutter hub and blade assembly used in an underwater pelletizer in which a molten polymer is extruded as strands through orifices in a die plate into a water box for cooling and solidifying, and cutter blades mounted on a rotating cutter hub move along the face of the die plate and cut the extruded strands into pellets. In this invention, the cutter blades are generally rectangular in configuration with the end edges of each blade being inclined or beveled to provide a cutting edge on each end edge of the blade. The blades are mounted on the cutter hub at a steep angle to the die face.
2. Description of the Prior Art
The use of a cutter hub with a plurality of blades mounted thereon in association with the die face of a die plate in an underwater pelletizer is well known. U.S. Pat. No. 4,123,307 discloses initial developments of a cutter hub supporting a plurality of replaceable cutter blades having a cutting edge associated with the die face of an underwater pelletizer to cut extruded strands of polymer into pellets in which the cutter hub is supported and rotated by a drive shaft with the die face, cutter hub and associated blades being oriented in a water box. Subsequent developments in this field of endeavor are disclosed in U.S. Pat. No. 4,251,198 in which the cutter blades are generally rectangular with the elongated side edges thereof being beveled and/or inclined and support arms on the cutter hub supporting substantially the entire length of the blades. The side edges of the blades are oppositely inclined so that the cutting edge on each side can be used by reversing the position of the blades on the cutter hub arms.
Published US Patent Application No. U.S. 2003/0102395 A1 also discloses an underwater pelletizer cutter hub with separator bodies mounted on the arms. The separator bodies are formed as rectangular impact slats mounted in an indentation substantially parallel to the axis of the drive shaft. This structure results in the separation bodies having a cutting edge perpendicular to the die face of the die plate. In this publication, the impact slats or separation bodies can be chamferred on their edge pointing toward the cut off head. This publication also indicates that the mounting of the separation bodies are such that any substantial inclination of the separation bodies relative to the drive shaft is avoided with a maximum inclination not to exceed an angle of 10°. It is not clear whether the maximum inclination of 10° of the separation body in relation to the rotational axis of the drive shaft orients the face of the separation body in relation to the die face as the separation body approaches the extruded strand at 80° or 100°. It is also not clear whether the chamfer is on the leading edge or trailing edge of the separation body as it approaches the extruded strand and impacts the extruded strand to separate a pellet from the strand.
In addition to the above prior art, cutter blades of a rectangular configuration having oppositely inclined or beveled cutting edges have been commercially available in which the inclination of the opposing blade cutting edges is about 45° with the inclination being opposite on the cutting edges to enable both inclined edges to be associated with cutter arms and the die face. Blades of this type are mounted on cutter hub arms to support the blades in standardized angular positions within the underwater pelletizer art. It is customary in this art to support rectangular blades having a 45° blade cutting edge angle at a 49° blade support angle, i.e., the angle between the planar surfaces of the blade and the die face surface. This leaves an included angle between the blade cutting edge and the die face of 4°.
In another standard cutter hub and rectangular cutting blade assembly, the blade cutting edge angle is 30° to the planar surfaces of the blade and the blade support angle is also at 30°, thus leaving no clearance between the blade cutting edge and the die face surface. Blades at support angles of 30° or 49° occupy a circumferential space around the axis of the cutter hub and drive shaft that limits the number of cutter hub arms and blades that can be used. One example of a commercial cutter hub and blades using the 49° blade support angle limits the number of blades to 12.